Normal values
Reference intervals vary slightly between laboratories and analysers. Read every result against the range printed on your own report.
| Value | Normal range | What it tells you |
|---|---|---|
| pH | 7.35–7.45 | Is the blood acidaemic or alkalaemic? |
| PaCO2 | 4.7–6.0 kPa (35–45 mmHg) | The respiratory component: CO2 is an acid the lungs blow off |
| HCO3− | 22–26 mmol/L | The metabolic component: the main buffer, controlled by the kidneys |
| Base excess | −2 to +2 mmol/L | Another view of the metabolic component; below −2 means metabolic acidosis |
| PaO2 | 10–13 kPa (75–100 mmHg) on room air | Oxygenation — always read against the inspired oxygen |
| Lactate | below 2 mmol/L | Tissue hypoperfusion; the number that matters most in a sick surgical patient |
To convert pressures: kPa × 7.5 = mmHg, and mmHg ÷ 7.5 = kPa.
The five steps
Step 1 — Look at the patient, then at the oxygen
Before any number, ask how unwell the patient is and how much oxygen they are breathing. A PaO2 of 12 kPa is normal on room air and alarming on 60% oxygen.
A rough rule: the PaO2 in kPa should be about 10 less than the inspired oxygen percentage. On 40% oxygen, expect roughly 30 kPa. Much lower means a gas-exchange problem even if the number looks “normal”.
Respiratory failure is a PaO2 below 8 kPa (60 mmHg). Type 1: the PaCO2 is normal or low (pneumonia, pulmonary embolism, oedema). Type 2: the PaCO2 is above 6.0 kPa — the patient is not breathing enough (opioids, exhaustion, COPD, a weak chest after a laparotomy).
Step 2 — The pH: acidaemia or alkalaemia?
Below 7.35 is acidaemia; above 7.45 is alkalaemia. A normal pH does not mean a normal gas — it may be a fully compensated or a mixed disorder (steps 4 and 5 will tell you).
Step 3 — Which one explains the pH: the CO2 or the HCO3?
Find the value that has moved in the direction that causes the pH:
- Acidaemia + high PaCO2 → respiratory acidosis
- Acidaemia + low HCO3 → metabolic acidosis
- Alkalaemia + low PaCO2 → respiratory alkalosis
- Alkalaemia + high HCO3 → metabolic alkalosis
If both explain the pH (high CO2 and low HCO3 in an acidaemic patient), there are two primary disorders at once — a mixed acidosis, and usually a very sick patient.
Step 4 — Is the compensation what you would expect?
The body pushes the other component in the same direction to pull the pH back towards normal. Compensation never overshoots and is rarely complete. If the second value is not where the formula predicts, there is a second disorder.
| Primary disorder | Expected compensation |
|---|---|
| Metabolic acidosis | Expected PaCO2 (mmHg) = 1.5 × HCO3 + 8 (± 2) — Winter’s formula |
| Metabolic alkalosis | Expected PaCO2 (mmHg) = 0.7 × HCO3 + 21 (± 2) |
| Respiratory acidosis | HCO3 rises 1 per 10 mmHg rise in PaCO2 if acute; about 3.5 per 10 if chronic (after 3–5 days) |
| Respiratory alkalosis | HCO3 falls 2 per 10 mmHg fall in PaCO2 if acute; about 5 per 10 if chronic |
In a metabolic acidosis, a PaCO2 higher than Winter’s formula predicts means the patient cannot breathe fast enough to compensate. In a surgical patient that is often exhaustion — and a sign to call for help early.
Step 5 — In any metabolic acidosis, calculate the anion gap
Anion gap = Na+ − (Cl− + HCO3−)
Normal is about 8–12 mmol/L (check your laboratory). Albumin is the main unmeasured anion, so a low albumin hides a raised gap: add 0.25 for every 1 g/L that the albumin is below 40. Most surgical inpatients have a low albumin, so this correction matters.
Causes of a metabolic acidosis
| High anion gap | Normal anion gap (hyperchloraemic) |
|---|---|
|
|
The delta ratio: is there a second metabolic disorder?
When the anion gap is high, compare how far the gap has risen with how far the bicarbonate has fallen:
Delta ratio = (anion gap − 12) ÷ (24 − HCO3)
| Delta ratio | Interpretation |
|---|---|
| below 0.4 | Normal anion gap acidosis |
| 0.4–0.8 | High and normal anion gap acidosis together |
| 0.8–2 | Pure high anion gap acidosis |
| above 2 | High anion gap acidosis plus a metabolic alkalosis (for example, a septic patient who has also been vomiting) |
ABG patterns on a surgical ward
| Patient | Typical gas | Why |
|---|---|---|
| Gastric outlet obstruction, persistent vomiting | Hypochloraemic, hypokalaemic metabolic alkalosis | Loss of HCl; the kidney holds sodium and loses H+ and K+ (paradoxical aciduria). Treat with 0.9% saline and potassium. |
| Ischaemic bowel, sepsis, shock | High anion gap metabolic acidosis, raised lactate | Anaerobic metabolism. A rising lactate after an operation should prompt a search for ischaemia or a leak. |
| High-output ileostomy or fistula | Normal anion gap metabolic acidosis, often hypokalaemic | Bicarbonate-rich small-bowel fluid is lost. |
| Large-volume 0.9% saline resuscitation | Normal anion gap, hyperchloraemic acidosis | Saline contains 154 mmol/L chloride. A balanced crystalloid avoids it. |
| Over-sedated after surgery (opioids, PCA) | Acute respiratory acidosis, type 2 failure | Hypoventilation. Check the respiratory rate and pupils; consider naloxone. |
| Pain, anxiety, early sepsis, pulmonary embolism | Respiratory alkalosis, sometimes with a low PaO2 | Hyperventilation. A post-operative patient with a low CO2, a low O2 and a tachycardia has a PE until proven otherwise. |
| DKA presenting with abdominal pain | High anion gap metabolic acidosis, ketones | DKA can mimic an acute abdomen. Check glucose and ketones before taking a patient to theatre. |
Five traps
- A normal pH is not a normal gas. Always check the CO2, the bicarbonate and the anion gap.
- Do not read the PaO2 without the FiO2.
- Compensation does not overshoot. If it seems to, there is a second disorder.
- Correct the anion gap for albumin, or you will miss a lactic acidosis in a malnourished patient.
- Venous gases are fine for pH, bicarbonate, lactate and electrolytes, but not for oxygenation; venous CO2 runs a little higher than arterial.
A worked example
Day 3 after a right hemicolectomy, confused and tachycardic
On 28% oxygen: pH 7.24, PaCO2 3.9 kPa (29 mmHg), HCO3 12, Na 136, Cl 100, albumin 28 g/L, lactate 6.5.
- Patient and oxygen: unwell; assess with ABCDE while the gas is analysed.
- pH 7.24: acidaemia.
- HCO3 12 explains it: metabolic acidosis.
- Winter’s: 1.5 × 12 + 8 = 26 mmHg (24–28). The PaCO2 is 29 — slightly above the expected range, so he is not fully keeping up: he is tiring.
- Anion gap: 136 − (100 + 12) = 24, corrected for albumin 24 + 0.25 × 12 = 27. High anion gap; with a lactate of 6.5 this is a lactic acidosis. Delta ratio (27 − 12) ÷ (24 − 12) = 1.25: a pure high anion gap acidosis.
Conclusion: a high anion gap (lactic) metabolic acidosis with inadequate respiratory compensation. On day 3 after a bowel anastomosis, think anastomotic leak — senior review, cultures, antibiotics, fluids and a CT scan.
Practise now
Coming next in the series
Acid–Base Balance: A Visual Guide for Medical Students — the illustrated book behind this page, free to download with a DOI, like the rest of the series.